Silicon, germanium, zirconium, and titanium are metals often used in chloride form as vaporous reactants for forming high purity metal oxide glasses. A pernicious by-product of such reactants is hydrochloric acid (HCl). The corrosive nature of HCl necessitates periodic replacement of the equipment having had prolonged contact with the acid. In addition, the enhanced global sensitivity to environmental protection has led to more strict government regulation of point source emissions; thus it is no longer acceptable to release environmentally pernicious gases, such as HCl, into the atmosphere without abatement.
Compliance with such standards has burdened industry with the task of containing, and properly disposing of, such pollutants. At present, vaporous exhausts, such as HCl, are treated with elaborate, and very expensive, cleansing systems that remove particulate matter from exhaust gases before they are released into the atmosphere. In addition, the periodic maintenance of equipment necessitated by the corrosive nature of HCl utilizes thousands of man-hours, as well as the consequential loss of manufacturing productivity.
The inventive solution proposed in U.S. Pat. Application Ser. No. 07/568,230, filed Aug. 16, 1990, now U.S. Pat. No. 5,043,002, is to use halide-free feedstocks, preferably a polymethylcyclosiloxane, and most preferably octamethylcyclotetrasiloxane (OMCTS), in these processes for producing metal oxides, thus eliminating the production of HCl as a by-product. The advantages of operating under a chloride-free system include: reduced pollution abatement requirements; reduced equipment losses and maintenance due to the corrosive nature of HCl; and increased manufacturing productivity. Likewise, the long term, adverse environmental effects inherent in producing large boules of high purity metal oxide glass or large blanks of soot from which optical fiber is drawn, are reduced significantly as fused silica glass produced by oxidation of OMCTS results in the production of carbon dioxide and water as essentially the sole by-products.
The instant invention is directed toward the doping of metal oxide glasses, particularly glasses used in the fabrication of optical fiber, by oxidation or flame hydrolysis of a halide-free, organometallic compound in vapor form. In this manner, metal oxide glasses may be doped in the manner described by Dobbins et al., while still catering to the environmental concerns addressed therein. Therefore, it is an object of the instant invention to provide a chloride-free, organometallic compound that is suitable for doping metal oxide glasses.
Optical fiber having one or more outer layers doped with titania has been shown to exhibit superior strength, as compared to homogeneous silica clad fibers. Superior strength is desirable in optical fiber as it reduces the potential for rupture and consequential replacement. It is a further object of the instant invention to provide a method of making doped optical fiber with superior strength as compared to standard silica clad optical waveguides.
Conventional methods for producing fiber clad with titania-doped outer layers utilize what is described in the art as a bump layer. For example, in U.S. Pat. Application Ser. No. 07/456,141 entitled METHOD OF MANUFACTURING OPTICAL WAVEGUIDE FIBER WITH TITANIA-SILICA OUTER CLADDING, filed Dec. 22, 1989 by Backer et al. now U.S. Pat. No. 5,067,975, and U.S. Pat. Application Ser. No. 07/456,140 entitled OPTICAL WAVEGUIDE FIBER WITH TITANIA-SILICA OUTER CLADDING, also filed Dec. 22, 1989 by Backer et al., a method of making a fatigue resistant optical fiber is disclosed. These applications describe an optical fiber having a core, an inner cladding, and two outer claddings, one nominally about 8% by weight TiO.sub.2 and a bump layer, with said bump layer comprising a TiO.sub.2 concentration greater that 10.5 per cent by weight. The sole purpose of the bump layer is to control the loss of titania during consolidation. The present inventive technique eliminates the need for the bump layer without compromising the strength of the fiber. Said technique may also result in improved select rates of drawn optical fiber.
The teachings of the instant invention are easily adapted to known methods of producing high purity fused silica by flame pyrolysis or hydrolysis, such as those disclosed in the early patents by Nordberg (U.S. Pat. No. 2,239,551) in 1941 and Hyde (U.S Pat. 2,272,342) in 1942. It is anticipated that this process alteration may be adapted to a variety of deposition/collection techniques as well.
While it is recognized that the primary application of the instant invention relates to the doping of fused silica, the technology applied herein is generally applicable in instances where a high purity metal oxide glass is desired.